Saturday, December 22, 2012

The capability to read analog inputs is a feature that is greatly missed on the Raspberry Pi, but I agree with the decision to omit this capability in order to keep the price down. Besides, if they did include an analog interface, many would complain that it isn't adequate for their purpose. How many input channels do you need? What resolution? 8 bit, 10 bit, 12 bit 16 bit? What throughput rate?

Fortunately, there are many analog input chips that use the SPI or I2C bus, making it almost trivial to add analog inputs to the Pi. I chose the MCP3008, an 8 channel 10 bit ADC available from Adafruit.com. Add a bi-directional logic level converter and some connectors and you're ready to go.

The level converter is on my main interface board which provides two SPI bus connectors. The analog interface is a simple board which includes a connector for SPI, the MCP3008 chip, a jumper to choose the analog reference, and screw terminals for the inputs. I ended up adding several more screw terminals connected to 5V in order to power thermistors.

The SPI serial bus is full duplex, but the way it works may seems odd to a programmers point of view. (It makes perfect sense if you understand how the hardware works.) You may be familiar with how full duplex works on an RS-232 line: data can be sent and received at the same time, but independently. That independence is due to the fact that RS-232 is an asynchronous protocol. SPI is a synchronous protocol; meaning everything is driven by a clock pulse.
Data bits will be sent out the MISO line on each cycle of the CLOCK line. At the same time data bits are being read in on the MOSI line. The number of bits out is the number of bits you will read back in. This means that you may have to write more bits than expected for a given command and you may read bits that are unused.

Fortunately, we don't have to worry much about the ugly details at the lower level. There is a device driver for SPI that is included with the recent versions of Raspbian and the WiringPi library provides support for SPI I/O. The functions wiringPiSPISetup and wiringPiSPIDataRW are all that is needed. Here is the source code for a program that I used when testing and calibrating sensors.

Note: The SPI device is not loaded by default. The easy way to get it loaded is to used the "gpio" utility that comes with the WiringPi library. Just entergpio load spi
and the drivers will be loaded and ready to use.

Tuesday, December 4, 2012

You may notice in the picture of my setup (in the previous post) that the Raspberry Pi is being powered normally via the USB power connector. This was done because the Pi was failing to boot when powered through the GPIO port as I had planned. While I did not do any tests, the obvious cause is a lack of sufficient current to power the Pi. It really does want to have a solid 700mA of 5 volt juice. My planned configuration worked fine on the workbench but failed once I mounted it on the wall and connected the alarm sensors. The sensors require 12V power and I was using the same power supply to drive the sensors and the power adapter for the interface board. That 12V power supply was only 2A and that was apparently too little for all of this.

I could have gotten a larger 12V power supply, but I already have several of these 2A supplies. Realization finally struck me that using a single power supply would be a mistake. If any of the sensor lines was compromised, shorting the power to ground, it would cause the Pi to shut down suddenly. So now I have one supply just for the alarm sensors and another that drives the interface board, and through that, the Pi.

The system is up and has been running for over a week with no major problems. I spent several hours improving my code for driving the X10 interface to make it as reliable as possible. It seems to work as well as it ever has when using heyu or ActiveHome to drive it. X10 is inherently unreliable, but is usually good enough for casual use. If you have a fairly new house that is wired properly, then it can work fairly well. My house, unfortunately, is old and poorly wired. I have made some improvements over the years, but there are still areas of my house that the X10 signals simply will not reach. I will cover X10 in more detail in an future post.

Tuesday, November 27, 2012

The interface is complete now with all (well, most anyway) of the kinks out of it. Here it is mounted on the wall in a utility area of the house.

I still need to install a few more sensors for the alarm system and I will soon add analog input using the SPI bus. That worked just as expected on the breadboard. You can see it connected in the blog post below.

There was some discussion on the forum about the serial connection for the X10 interface. My CM11a works fine with just Tx, Rx, and GND connected.

The power supply was salvaged from a micro ATX PC case. The 12V power supply that powers it is only 2A and I had trouble running the RasPi powered from my interface. It worked fine on the bench with these same power supplies. I assume that the motion detectors, which are wired to the same 12V supply are drawing too much current. This causes the ATX PS to fail to output adequate 5V current. For the time being I have removed the power jumper (+1 for configurability) and just power it the normal way. Once I get a beefier 12V PS, I will try powering the RasPi from the interface again.

Monday, November 5, 2012

After yet another design change, I finally have the interface complete and ready to test.

Version 7 Finally Comes to Life

✓ Serial Port✓ 8 Digital Inputs

✓ 4 Relay Outputs

✓ 2 SPI Ports

✓ 1 I2C Port

✓ Fused Power to Pi

Making the fuse involved the tiniest soldering I have ever done. That is a surface mount poly-fuse soldered to two wires. Once connectors are added, it is used for the jumper that connects the interface board 5V supply with the Raspberry Pi GPIO 5V pin.

The picture above shows one SPI port connected to an ADC chip on the bread board. Once everything checks out, it will be time to mount it all.

I still need to to finish wiring the SPI connectors, but the serial port, the digital inputs, and the relays are all working. I will probably go ahead and add connectors for the I2C bus pins for future use. I already have an eight port A/D converter for the SPI bus and more GPIO would also be easy to add via SPI.

I added a jumper block near the GPIO connector that allows me to connect (or not) 5V on the interface to 5V on the RasPi. This would allow me to power the board from the RasPi or to backfeed power to the RasPi from the board. I plan on making a jumper with a polyfuse inline. Using that as the jumper will prevent the RasPi from drawing too much current.

Monday, October 8, 2012

Edit 15-Oct-2012: Posted corrected code - missed a few typos. GpioPoller.c is now the multiplexed version. Edit to make HTML behave nicely.

I have been looking at this source code issue from the wrong perspective. I knew that I would be posting my source code here eventually, but I didn't think that it would be useful to that many people. That was when tunnel vision had me thinking of just this alarm system project.

The example code that I present here is really much more widely applicable. This is my main function, which implements a daemon process in C. Also included are my data structures and my method for using worker threads. Copious comments have been added to help clarify things.

I printed it on heavy card stock and on regular paper. The regular paper was used for practice. Good idea since I found that I folded it upside down. Fold it with the printed side down. I used scissors, an exacto knife and an old piece of wood as a cutting board. Here is how it went.

All it takes - printed card stock, scissors, and a knife.

After being cut out.

Make good creases on all the folds.

The result - Very functional and surprisingly study little box.

The connectors and SD card hold the RasPi tightly in place.

I like the result. Especially for a case that is basically free (assuming you can get a sheet of heavy card stock.) The plain white looks really boring, but it should be pretty easy to add whatever design you like.

Flip the case over and there is lots of space to express your inner Pi. OK, so I'm not Picasso.

Tuesday, October 2, 2012

I have used the relay circuit from my earlier post many
times and I know that it works. However,
I have seen that circuit drawn with a 1KΩ resistor and also with a 10KΩ
resistor and started to wonder why. Since this blog is intended to help new
hobbyist learn, I thought it would be good to share what I found out. (Warning:
There is math involved )

After some thought and further research I made the following
changes to the GPIO relay control circuit.
The current limiting resistor is now 10K and I will explain why in a
moment. Also, a 100KΩ resistor has been
added. This is recommended to ensure
that the transistor turns off if the input is left open. In my design, the input is always either high
or low and is never open, so I am leaving this out of my interface.

The current limiting resistor must be sized to allow enough
current to saturate the base of the transistor to guarantee that it
switches. Current too far above this
level could damage the transistor. Every
transistor has an inherent property known as the common emitter current gain,
commonly referred to as HFE. The proper
size for the resistor may be calculated using the following formula.

R = Supply Voltage /
( max A / HFE * 1.3 )

The supply voltage (from the GPIO pin) is 3.3V.
The HFE for a 2N2222 transistor is assumed to be at least 100. The current draw on the relays that I use
ranges between 30 and 50mA. Plugging in
the numbers for 30mA gives a result of R = 8461. If 50mA is used, then R = 5076. However, the HFE is very likely higher. If we assume that it is 150, then the resistor value at 30mA becomes 12692. This shows that a 10KΩ resistor is probably more
appropriate for this configuration.

The bottom line is that any resistor between 1K and 10K should work OK. I just felt that this issue should be addressed before any arguments start.

Saturday, September 22, 2012

I keep coming up with so many improvements to my interface design (I'm up to version 6 now) that I'm not getting much built. I decided that I had to make the pins for the SPI and I2C buses available. But that means using five GPIO pins that I had other plans for. I have to have at least eight inputs to handle the motion detectors and door sensors for the alarm system. At least four relay outputs would be nice too, so the Raspberry Pi is quickly running out of GPIO pins.

Time to get a little help from another useful IC - the multiplexer. There are several possible choices, but the obvious one is the 74151, of which I conveniently happen to have a few.

This diagram shows how this needs to be connected. GPIO pins 1, 2, and 3 are set to output mode and used as address lines to select which input to read. GPIO pin 0 is set to input mode and connected to the output of the multiplexer.

(Note: I always refer to WiringPi pin numbers, not the standard BCM numbering.)

The result is that the interface can still have 8 inputs, but only use 4 GPIO pins to do it.

I will probably do the same thing (in reverse) to allow two GPIO pins to provide four select lines for the SPI bus. It may be a while until I do that since the SPI bus is for future expansion. I don't have any devices for it yet, just ideas.

Thursday, September 20, 2012

To allow the greatest flexibility, my outputs are all relays. This allows me to switch a variety of voltages and provides protection to the GPIO pins of the Raspberry Pi. A relay is just a switch that is controlled by an electromagnetic coil. Powering the coil will make the switch turn on. The relays I used are made to mount on a circuit board and can be driven by 5V. They can easily switch 12V or more at a moderate current level. They are NOT meant to control house current! That can be the topic of another post.

The Pi GPIO pins will only output 3.3V at a few milliamps. This is not enough to drive the relay directly, but is is enough to switch a transistor on and off. A common NPN switching transistor handles that job nicely. I used the 2N2222 which is highly available (i.e. even Radio Shack carries it.)

Note: NO SPST is a switch type designation and means Normally Open, Single Pole Single Throw. This is the simplest type of switch.

A 1K ohm current limiting resistor is attached to the base of the transistor. Power is connected to the relay coil which is then connected to the collector of the transistor. The emitter is connected to ground. When the GPIO pin is low, no current will flow from the collector to the emitter and the relay will be off. Setting the GPIO pin to high will "turn on" the transistor, power will flow and the relay will turn on.

Notice that there is a diode attached across the coil of the relay in a reversed orientation. There is magnetic energy stored in the coil while it is energized and holding the relay closed. When power is removed, the collapsing magnetic field in the coil causes a brief but powerful surge of reverse voltage which can damage the switching transistor and cause premature failure. The diode is there to stop this reverse voltage and protect the transistor. I prefer to use relays that have internal surge suppression diodes and they are becoming more common now. If your relay does not have the diode internally, then it is highly recommended that one be added.

Wednesday, September 19, 2012

After holidays and many other distractions, I am finally able to get back to work on my Raspberry Pi interface.

Here is the GPIO input circuit that I came up with using an opto-coupler for protection. The opto-coupler that I chose is the LTV-847 (Jameco part number 878286) which provides 4 opto-couplers in a 16-pin DIP format.

Power applied to the anode and cathode will cause the internal LED to emit light. This is detected by the internal photocell which controls the output. Because there is no electrical connection between the input and output sides, opto-couplers are handy for connecting between very different voltage levels. They are also excellent at preventing the introduction of electrical noise into a system. For this application, the isolation will provide electrical protection to the Raspberry Pi.

The complete circuit for using this is shown below. The 1KΩ resistor on the input is for limiting the current that can flow through the LED. The 10KΩ pull-up resistor is internal to the Raspberry Pi. Be sure to set the pull-up option when you set the pin to input mode. Using a separate 5V power supply for the interface provides greater protection than powering this all from the 5V line on the GPIO header. If that line gets shorted to ground, or even if it just draws too much current, it can cause the Pi to suddenly reboot.

When the input is open, no current will flow through the detector and the Raspberry Pi will see the pin hi due to the pull-up resistor. When the the input is connected to ground, current will flow and the Pi will see the the pin as low, since it is effectively connected to ground now.

Tuesday, August 28, 2012

If you read any about interfacing to the Pi GPIO pins you are bound to come across the dire warnings - BeCareful. These pins connect directly to the microprocessor without any protection. You can destroy the Universe if you wire something wrong.
OK. Maybe not that dire, but you get the idea. You may get away with interfacing directly with the GPIO pins (I know I have) but even if your circuit is designed perfectly, accidents still happen. Something can fall across the circuit board and short things out. So it is wise to protect your Pi.

I am building an interface that will connect to hard-wired alarm circuitry. That means lots of lines running lots of places and just that many more opportunities for bad things to happen. This protection is also a good idea since a lot of people using the Pi will likely be from the younger and less experienced crowd. This is what the Pi was developed to encourage, so having a safe way to interface to the most flexible I/O on the device is critical.

Here are some of the options for protecting a logic circuit.Zener Diode - A Zener diode is one that allows no reverse current to flow until a threshold is reached. Above that threshold, current will flow. A 3.3V Zener diode between a GPIO pin and ground can protect it from any over voltage that is applied. Any voltage above 3.3V will just be shunted to ground.Mike Cooke has provided a design for a screw terminal break-out board for the Raspberry Pi GPIO that uses Zener diodes for protection. The design may be found here: Raspberry Pi Breakout Box. Transistor - A transistor can be used to switch a known safe logic level.Line Driver or Buffer - Chips that contain multiple transistor switches internally. These are easier to install and provide a cleaner design when you have many lines to protect.Opto-Isolators - Chips similar to Buffers but these use pairs of internal LEDs and optical sensors instead of transistors. This provides total circuit isolation and can be useful in a noisy electrical environment.

A Bi-directional Logic Level Converter seemed like the perfect thing to use. However, I tried the TXB0108 provided by Adafruit and had problems. It seemed to work fine in my prototyping board, but when I put things together, I found that it had problems driving some TTL chips. I checked it with a multimeter and found it only put out 2.5 volts when asserted on the 5V side. This wasn't enough for an input into the MAX232N that is used for the RS-232 interface.I am interested in hearing from anyone else who has tried this chip.The GPIO pins that are used for output on my interface are protected by transistors which control relays. That is more than adequate protection. I am still pondering what route to take now for protecting the inputs. Opto-isolators are something I am already familiar with (and probably already have some) so I am leaning toward that option.Eventually, I will build a new version of my interface so that I can apply all that I have learned along the way. I already regret not leaving pins open for I2C. That will have go in the next revision.

Monday, August 27, 2012

Many of the GPIO pins on the Pi have other special uses. The most useful of these are the serial port pins #8 and #10, which are transmit and receive for an RS-232 serial port. By default this port will output diagnostic messages during boot and then provide a user login. The configuration is 8 bits, no parity, 1 stop bit, no hardware handshaking, at 115200 baud. The device name is /dev/ttyAMA0.

I need to use this serial port to interface to my X10 system via a CM11A computer interface module. That can be a topic for several future posts.

First lets cover some important facts about RS-232 and voltage levels. The Pi uses levels that are 0V to represent a zero and 3.3V to represent a one. RS-232 uses -3V to -15V to represent a zero and 3V to 15V to represent a one. Thus, a level converter is required to create this interface. The MAX232 series of chips was designed for this exact purpose.

External capacitors are needed to drive the charge pumps inside the chip. Note: There are several variations of the MAX232 chip which have different requirements. The one shown in the circuit here uses 0.1uF capacitors. The ones I have use 1.0uF and some versions even have the capacitors built in. When in doubt, check the datasheet for the chart that shows the requirements for each variation.Data Sheet for MAX232 family

If, like me, you want to take complete control of the serial port for your own uses, there are two configuration changes to make:

First, disable the boot up and diagnostic output to the serial port.sudo vi /boot/cmdline.txt
and remove the two options referring to the serial port.
So, thisdwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait

The system booted up just as expected. I logged on and entered "startx" to begin the graphical interface. Gave it a test drive. The processing power is on the low side but still pretty good. A powerful graphics chip makes 1080p HD video possible. I am impressed.

Ran the configuration programsudo Raspi-config
set timezone and enable SSH

The Raspberry Pi Board

The Raspberry Pi is a credit card sized single-board computer developed in the UK by the Raspberry Pi Foundation and based on the Broadcom BCM2835 system on a chip. This US$35 board is intended to stimulate the teaching of basic computer science in schools. It's also a great item for hobbyists.

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About Me

I'm a long time electronics hobbyist with a great interest in the Arduino microcontroller and the Raspberry Pi single board computer. Both are great for teaching kids about electronics and computers, which is my other great hobby.
These blogs will be used to document and share my projects for students and fellow hobbyists.